Tel：
+86 13143444311
Email：sst@szsst88.com
Fax：+0086-755-27322278
Address：Building 9, First Industrial Zone, Tantou Industrial City, Songgang Street, Baoan District, Shenzhen 518105, Guangdong, China
Type:Stainless Cap Screws
Materials:Stainless Steel
Surface treatment:Plain
Head Type:Cap Head
Drive:Per Drawing
Thread:Machine Thread
Point:Blunt
Forming:Single Die Heading
Size:M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M14
Number:540
We make stainless cap screws in passivated A2 and A4 with a cylindrical head, a socket and a machine thread, quoting them from M1 to M14 to the standard your drawing names. Head height, socket, thread and the pocket they seat in are reviewed together, and the first article is seated in your own counterbore.
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ISO 9001:2015 · Manufacturing since 1998 · 8,678+ customers · 89 countries and regions
Made to order – there is no stock catalogue. Each part is produced to your drawing, and production starts after drawing review and first-article approval.
Stainless cap screws are socket-head screws with a cylindrical head and a full flat bearing face, made in stainless so the joint survives weather and washing. The recorded item is stainless with a plain finish, a cap head and a machine thread, quoted from M1 to M14 with Number 540.
Cap head. A straight cylinder with a flat top, a chamfered edge and a flat bearing face. It is designed to sit inside a counterbore, so the head is buried and the bearing face is what the joint sees.
Socket drive. An internal hexagon or a six-lobe star, depending on the drawing. Both are cut into the top face and both depend on a key that seats fully to carry torque.
Machine thread. A rolled thread with a blunt chamfered end, made for a tapped hole or a nut.
Stainless, plain. Passivated and uncoated, so the head, the socket and the bearing face are exactly as formed.
Stainless cap screws only work as designed if the part they go into has a counterbore: a flat-bottomed pocket deeper than the head is tall, with a diameter that clears the head chamfer. Without it, the head stands proud, the bearing face lands on the edge of a hole, and the screw behaves like a machine screw with a bad head.
On stainless cap screws the depth of that pocket is the dimension that gets argued about. A counterbore exactly the head height leaves the head flush; a little deeper leaves it below the surface, which is what a machine table or a sliding face needs; a counterbore that is too shallow makes the head stop on the top of the pocket instead of on its floor, and the joint is then held by a ring at the head edge rather than by the full face.
On stainless cap screws the counterbore diameter has to clear the chamfer on the head. A pocket cut to the nominal head diameter leaves the chamfer touching its wall, and the head then jams before it seats. That is why the standard gives a clearance diameter rather than the head size, and why a cap screw in a tight pocket is a fit problem that looks like a thread problem.
On stainless cap screws the corner at the bottom of the pocket matters. The head-to-shank transition on a cap screw is a radius, and a pocket with a sharp internal corner presses that radius into the corner as the screw is tightened. On a stainless part that contact can gall, and a galled head-shank radius is a screw that will not come out without damaging the pocket.
Thin plate is the limit. A counterbore removes material, so a plate that is only a little thicker than the head leaves very little underneath it, and the head then pulls a ring of plate out when the joint is tightened. Where the plate is thin, a shallower head or a washer under a pan head is the answer, not a deeper pocket.
The family is quoted from M1 to M14, with M1, M2, M3, M4, M5, M6, M7, M8, M9, M10 and M14 in the recorded range. Custom lengths up to 2,000 mm are subject to DFM review after drawing submission; on stainless cap screws the head diameter and height, the socket size and the counterbore are quoted together, because all four come from the same standard.
| Item | How it is defined | What we control |
|---|---|---|
| Head | Cap head, flat face | Diameter, height and face flatness |
| Socket | Hexagon or six-lobe | Size and depth, key tried on the part |
| Thread | Machine thread | Pitch and length, gauged per batch |
| Counterbore | Clearance diameter, depth | Matched to the head on the first article |
| Finish | Plain stainless, passivated | Surface condition and film |
| Length | From under the head to the end | Measured on the shop plate |
The head height and the counterbore depth are quoted together because either one alone says nothing. A drawing that gives a head height without the pocket leaves the customer to discover the fit on the bench; one that gives a pocket without the head height cannot be inspected at all.
Length is measured from under the head, because that face bears. In a counterbore the visible depth is the pocket depth plus whatever part of the head stands above the floor, so a screw that is right in length can still look wrong if the pocket is shallow.
Three features define stainless cap screws: the cylindrical head, the socket and the stainless class. The first is what makes it a buried fixing, the second is what allows it to be tightened inside that head, and the third is why it can be used where a steel screw would rust.
| Feature | What it gives | What it costs |
|---|---|---|
| Cap head | Full bearing face in a counterbore | Needs a counterbore, not a plain hole |
| Socket drive | High torque in a small head | Needs the matching key, and only that key |
| Stainless | No coating to chip at the face | Galls in a stainless pocket or nut |
| Flat bearing face | Load spread on the pocket floor | The floor has to be flat and hard |
| Plain finish | Socket and face as formed | Pitting in chloride at the socket |
The comparison with a pan head is the counterbore. A pan head sits on the surface and needs no pocket, which is cheaper and works on a thin plate; a cap head sits inside the part, which is what a working face, a sliding surface or a stacked assembly needs. The cost is the machining of the pocket and the loss of material under the head.
The comparison with a hexagon head is access. A cap screw is turned from above with a key and needs no clearance at the side, so it fits inside a raised boss or a deep pocket where a spanner cannot reach. The trade is that the socket is the only way in, and a rounded socket cannot be recovered by gripping the head.
The recorded finish is plain, passivated stainless. Nothing is added, so the head sits the same height in its pocket as the drawing says, and the bearing face is metal against metal rather than a film that compresses when the joint is tightened.
| Option | How it protects | Effect on socket and head | Typical use |
|---|---|---|---|
| A2 plain, passivated | Passive chromium oxide film | None; gauged as formed | General outdoor and process work |
| A4 plain, passivated | Passive film with molybdenum | None; gauged as formed | Coastal and chemical |
| Black oxide on stainless | Dark conversion film | Almost no change | Dark visible assemblies |
| Zinc plated steel | Sacrificial zinc | A few microns; both re-checked | Indoor, cost-driven work |
| Dacromet on steel | Zinc-aluminium flake layer | Film builds; socket re-checked | Outdoor steel structures |
In a pocket, the finish question is really a corrosion one. Water sits in a counterbore around the head, and that is a crevice where the passive film struggles to rebuild: a stainless cap screw in a wet pocket pits at the head-shank radius before it pits anywhere on the outside. A drain, a seal or a different pocket design does more than a coating would.
The other behaviour to design around is galling. A stainless head pulled down onto a stainless pocket floor under torque can seize at the contact face, and a cap screw that has seized in its pocket cannot be turned out with a key without damaging both. A lubricant on the thread, or a different material for the pocket, prevents it.
Three assemblies where stainless cap screws are the fixing:
| Where it sits | The screw clamps a jaw or a stop inside a counterbore so that the working face stays clear and nothing catches a workpiece. |
| When it goes wrong | A head that stands proud leaves a burr on the table, and a shallower pocket stops the head before its face bears. |
| Why it happens | The face is a datum, the head is buried, and the joint is re-set often. |
| What we control | Head height and pocket depth matched, socket driven with the production key. |
| Where it sits | The screw holds a plate or a fitting inside a pocket where the surface must be cleanable and the part is washed. |
| When it goes wrong | Water sits in the pocket, and a plated screw rusts at the head-shank radius where nobody looks. |
| Why it happens | The joint is wet, the head is buried, and the surface is inspected. |
| What we control | Grade confirmed, socket and head checked, passivation inspected. |
| Where it sits | The screw clamps a bracket or a fitting where the joint carries load, the head is buried in a countersunk pocket and the environment is wet. |
| When it goes wrong | A2 pits in chloride, and a seized stainless screw in a stainless plate has to be cut out. |
| Why it happens | The joint is loaded, the environment is wet, and the parts are expensive. |
| What we control | A4 grade confirmed, socket and thread checked, lubricant recommended. |
Four reasons that matter on stainless cap screws, each tied to what the drawing has to prove:
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Head, socket and pocket reviewed together
The head diameter and height, the socket size and the depth of the counterbore are reviewed as one set, so a head that would jam in its pocket or a socket too shallow for the key is raised before the batch is made. Basis: Head, socket and pocket reviewed as one set. |
Heading, socketting and passivation in one route
The cap head and the socket are formed in one stroke, the thread is rolled and the parts are passivated in the same route. Basis: One route from wire to passivated screw. |
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Key and pocket tried
The socket is turned with the production key and the head is seated in a sample pocket on the first article, because both features are proved in use. Basis: First article keyed and pocketed. |
First article fixes the setting
The first article fixes the wire heat, the socket punch and the passivation; repeat orders are compared against that record. Basis: First article signed off before the batch is released. |
Shi Shi Tong forms these stainless cap screws from stainless wire on single-die headers that form the cap head and the socket in one stroke, rolls the machine thread to a blunt chamfered end and passivates the parts. The first part of every run is measured on the head, turned with the production key and seated in a sample counterbore before the batch continues.
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The socket punch is the tool that limits the die. It wears at the corners or the lobe tips first, and a worn punch leaves a socket that is wider at the mouth than at the floor, so the key sits high and the load goes into the tips. On a buried head that failure is invisible until the key rounds it, because the head is inside the pocket and nobody looks at it until the joint has to come apart.
What decides whether stainless cap screws pass is the head, the socket, the thread and the alloy. The inspection agreement covers those, and the criteria come from the drawing.
Head and socket on the projector |
Thread gauged per batch |
Hardness checked against the grade |
Salt-spray testing of the grade |
The head is checked for diameter, height and face flatness, and the socket for size and depth; the first article is then seated in a sample counterbore and turned with the production key. A head that measures to the drawing can still jam in a pocket that was cut to the nominal diameter, which is why the pocket sample is part of the check.
The thread is gauged per batch, and the alloy is confirmed on the certificate. On a stainless part the socket is inspected for a rust bloom, which is a sign of free iron rather than of a wrong grade.
Every shipment of stainless cap screws is traceable to the wire it was made from and to the passivation batch. The grade and heat number are recorded on the material certificate supplied with the parts, and the inspection records follow the criteria agreed on the drawing.
ISO 9001:2015 |
RoHS test report |
Registered trademark |
Patent: security screw and driver |
With each batch we supply the material certificate, the inspection record for the criteria on the drawing and, where it is listed, the third-party report for the grade or the passivation. The documents carry the drawing revision and the batch number, so the paperwork can be matched to the parts.
More than 8,678 customers in 89 countries and regions have ordered from Shi Shi Tong; 87% of orders come from repeat customers and 56% through referrals. Most projects follow the same route: drawing review, first-article approval, then batches checked against that sample. On a buried head the first article is also seated in the customer’s pocket, because the pocket depth is what decides whether the head sits flush. Buyers visit during the process to review samples and to see how the heads and sockets are formed.
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8,678+
customers served
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87%
of orders from repeat customers
|
56%
of orders through referrals
|
89
countries and regions shipped to
|
Buyers and quality engineers visit for sample review, process discussion and supplier audits; the visit is arranged around production so the headers and the passivation line can be seen on the day the parts run.
Customer group visit – sample review |
Customer group visit – process discussion |
Customer group visit – factory tour |
Customer visit – project kick-off |
Danish customer – drawing review |
Industry exhibition – meeting customers |
Yes. A cap head is designed to sit in a flat-bottomed pocket that clears its chamfer. Without one the head stands proud and the bearing face lands on the edge of the hole.
Equal to the head height for a flush finish, or a little deeper to bury the head below the surface. A shallow pocket stops the head before its face bears.
Because the pocket was cut to the nominal head diameter rather than to a clearance diameter, so the head chamfer touches the pocket wall before the face seats.
It can, at the head-shank radius where it presses on the pocket corner. A lubricant on the thread, or a different material for the pocket, prevents it.
Only if the plate is thick enough to keep material under the pocket. Otherwise a shallower head or a washer under a pan head is the answer.
Head diameter and height, socket size and depth, thread size and length, length under the head, the pocket clearance and depth, and the stainless grade.
How a stainless cap screw project runs from drawing to repeat delivery:
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1
Send the drawing and the pocket detail
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→ |
2
Head, socket and pocket review
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→ |
3
First article seated in your pocket
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→ |
4
Batch production and repeat orders
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Each step is confirmed with you before the next one starts; the drawing revision from the first article is what repeat orders are checked against.
For a quotation, send the drawing with the head diameter and height, the socket size and depth, the thread size and length, the length under the head, the pocket clearance and depth, and the stainless grade.
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Head diameter
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Head height
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Socket size
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Thread and length
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Pocket depth
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Stainless grade
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Sending a drawing? The form below does not take attachments. Email the PDF, DXF or STEP file to sst@szsst88.com with the part name and quantity in the subject line – the drawing is reviewed and a quotation follows.
+86 13143444311
sst@szsst88.com
